The present invention relates to a signal modulator and related modulating method, and more particularly to an image-free signal modulator and related modulating method.
In a wireless communication system, such as a low IF (intermediate frequency) transceiving system or an OPLL (Offset Phase-locked loop) transceiving system, two oscillating signals (i.e., the in-phase oscillating signal and the quadrature-phase oscillating signal) having a 90 degree phase difference can be used to modulate/down-convert a radio frequency (RF) receiving signal via a frequency modulator in the receiver to obtain an in-phase input signal and a quadrature-phase input signal, and used to modulate/up-convert a base-band signal via a frequency modulator in the transmitter to obtain an up-converted in-phase signal and a up-converted quadrature-phase signal. When the phase difference between the two oscillating signals is not perfectly 90 degrees, however, some unwanted signal, e.g., the image signal, may be induced in the output of the modulator. Conventionally, the phase mismatch between the in-phase oscillating signal and the quadrature-phase oscillating signal is mainly caused by the length mismatch between the traces utilized for conducting the in-phase oscillating signal and the quadrature-phase oscillating signal respectively. Ideally, the problem of the image signal may be improved by carefully routing the layout of the traces to make them perfectly equal in length and parasitic condition. In practice, however, the intrinsic layout mismatch problem may still cause mismatch of the traces and inevitably generate the image signal. Since the image signal may seriously affect quality of the wireless communication system, such as the linearity, how to remove the image signal that emerges in the output of the frequency modulator in a wireless communication system has become an important issue in this field.